The 5th box is pushed upward onto the top conveyor, and then as it comes around you use the timer switch to get it into postion to take off, just like you did the others.
You have to use the timer switch to stop the box in the correct postion; you have a few seconds to push it down on to the other boxes.
Then go back and hit the timer again to give you more time so you can get over the conveyor yourself and then, put the box into the water hole to the right so you can walk off.
You cannot control the conveyor at the top, only the one at the bottom.